This two-arm randomized trial will provide digital cognitive behavioral therapy for insomnia (CBT-I) to all participants and passive body heating (PBH) sessions using a sauna blanket to half of participants over a 9-week treatment period. Participants are adults aged 18 years or older with insomnia disorder.

Insomnia disorder is a common and consequential mental health problem, with prevalence estimates at approximately 10%. The American College of Physicians recommends cognitive behavioral therapy for insomnia (CBT-I), an efficacious "mind" (psychological and behavioral) treatment, as the first-line treatment for adults with insomnia. Yet, about half of individuals experience residual insomnia symptoms after CBT-I. Accordingly, researchers have sought to augment CBT-I, primarily with pharmacotherapy, but many people do not want to use pharmacologic agents due to concerns about side effects and dependence. Passive-body heating (PBH), a "body" treatment, involves heating the body via hot baths or showers, infrared sauna, or other heat sources. PBH has been found to reduce the time needed to fall asleep and to improve sleep quality. PBH may improve sleep by increasing skin temperature and decreasing core body temperature, a dynamic associated with sleep onset. CBT-I and PBH thus target distinct factors that may contribute to insomnia, and the investigators hypothesize that the combination of CBT-I and PBH holds promise as a multi-component treatment for insomnia disorder.

Inclusion Criteria:

• 18 years or older
• English-speaking
• Willing to use birth control if assigned female sex at birth
• Willing to receive study text messages during participation
• Location to plug sauna blanket into regular wall outlet
• Ability to lie in sauna blanket for 15 minutes
• Ability to fit in the sauna blanket
• Daily access to the internet via computer, smartphone, or tablet
• Elevated insomnia symptoms as indexed by a score of 11 or greater on the Insomnia Severity Index (ISI) at two screening timepoints
• Primary insomnia disorder (characterized by difficulty initiating sleep) as indexed by a positive diagnosis on the Structured Clinical Interview for DSM-5 Sleep Disorders (SCISD)

Exclusion Criteria:

• Night shift or swing shift worker
• Current or planned routine body heating practices (e.g., saunas, hot tubs, long baths) within 4 hours before bed that are 10 minutes or more in duration (≥2x per week)
• Pregnant or plans to become pregnant during the participation period
• Traveling internationally between screening assessment and anticipated post-intervention assessment, or during anticipated post-follow-up assessment period (~1 week)
• Other diagnosed sleep disorders or suspected sleep disorders
• Medical conditions that might increase the risk of passive body heating using an infrared sauna blanket
• Medication use that might increase the risk of passive body heating using an infrared sauna blanket
• Mental health disorder that may better explain insomnia, require priority treatment, or be exacerbated by time in bed restriction
• Medical condition that may better explain insomnia, require priority treatment, or be exacerbated by time in bed restriction
• Lifetime diagnosis of a bipolar disorder, psychosis, or other psychotic disorder
• Current (past 3 months) major depressive disorder
• Current (past 12 months) suicidality, alcohol use disorder, other substance use disorder (except mild cannabis use disorder), panic disorder, agoraphobia, social anxiety disorder, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and eating disorders
• If using medications for mental health treatment (e.g., antidepressant medication) must have been on a stable dose for prior 8 weeks
• Currently receiving cognitive behavioral therapy for insomnia (CBT-I)

The final IPD dataset will include self-reported demographic data, other self-reported data (e.g., subjective sleep measures, mood symptoms), and wearable-derived sleep data (wearable-derived sleep data will be shared as nightly aggregates).

Beginning 3 months after the publication of results and unending.

Data will be accessible via the Vivli platform. To access IPD arising from this project, users must complete the Vivli data request form and sign the Vivli Data Use Agreement, which limits subsequent use to the terms of the approved request and requires that users maintain data security, and refrain from any attempts to reidentify research participants or engage in any unauthorized uses of the data. In order to get access to the data, the user must submit a valid scientific question, include a statistical analysis plan, and complete all required fields on the Vivli data request form. Vivli will review the data request for completeness. Anyone who has submitted an approved data request and signed a data use agreement on Vivli will be given access to the data. Vivli will then make the data available, without cost, to users for a set period of time. Vivli will maintain storage and access of the data for as long as it maintains scientific utility.

The clinical outcomes assessor who conducts the post-treatment clinical interviews will be masked to study randomization.